Air bleed valves are used in aircraft hydraulic systems to remove unwanted air from the hydraulic circuit prior to the operation of the high pressure hydraulic system to prevent unexpected and unwanted operational anomalies. Due to certain flight regimes, a traditional air bleed valve cannot be used in certain high performance aircraft, primarily those aircraft used in military applications. High G loads and inverted flight modes do not allow the air in the hydraulic system to be bled when experiencing these flight regimes. Therefore it is necessary to use sensors to determine when there is air in the hydraulic system and then electronically open an air vent valve to discharge the air when the aircraft is flying in a suitable flight mode. Traditional air bleed valves are usually bled when the pilot manually triggers the vent valve circuit. Sensors can be used in the air bleed valve such as a light emitting diode and a photoelectric diode to indicate that there is air in the hydraulic system and then send a signal to the pilot that the air vent in the air bleed valve needs to be activated. Pub. No. US 2010/0319791 A1 to Dirkin et al. disclose such a system. In the Dirkin system two LEDs and a phototransistor and three transparent windows are used to sense the presence of air. When air is detected by an electronic circuit which is connected to the phototransistor and the LEDs, a signal is sent to the flight deck so that the vent valve can be activated. This system is subject to several operational limitations involving clouding of the windows and failure of the phototransistor.
Other bleed air systems known in the art include those shown in U.S. Pat. Nos. 4,524,793 and 4,813,446 to Silverwater et al. These prior art devices provide for the automatic bleeding of air at the time of hydraulic pump start up using differential pressure between the air and the hydraulic oil to move a piston to control the bleeding process. This system is self activating and is not controlled by the flight crew or an electronic control system so the air is automatically vented whenever it is present irrespective of the aircraft flight mode. This presents a problem in high performance aircraft since the air cannot be vented in certain flight regimes. Also, this type of air bleed valve is not as reliable or dependable as what is needed in the industry for use in high performance aircraft.